The use of normal-density 14 to 15.8 lb/gal (1.7 to 1.9 g/cc) cementing slurries in completing geothermal and oil wells frequently results in problems of lost circulation when attempts are made to cement well regions that pass through weak unconsolidated rock zones having very fragile gradients. Specifically, these unconsolidated formations fracture as a result of the high hydrostatic pressures required to pump the highly dense cement slurries. To avoid this problem, high-temperature low-density cement slurries are needed to support the intermediate casing pipe and to protect the casing from corrosive fluids and gases.
One method for the preparation of lightweight cement slurries which can be used in geothermal and oil wells is to incorporate pressure-resistant hollow microspheres into cement slurries [Martin, Proceedings of the Third International Seminar of EC Geothermal Energy Research, Munich, Germany, p. 447, (1983)]. In addition to producing a lighweight slurry, the relatively high bridging ability of the hollow beads enhances its effectiveness in controlling lost-circulation problems.
However, experience with microsphere-filled lightweight cement systems suggests that the inclusion of the microspheres as a filler is not sufficient to yield the properties needed for geothermal cements. For instance, when a slurry with a density of 1.16 g/cc is autoclaved for 24 hours at a temperature of 300.degree. C. and a hydrostatic pressure of 1500 psi (10.3 MPa), the compressive strength of the set cement paste is only 610 psi (4.2 MPa). The low strength is presumed to be due to poor bonding at the cement-microsphere interface, and a low degree of hydration in the cement matrixes and the interfacial regions. Therefore, modification of the microsphere surfaces was necessary to produce a high-quality high temperature lightweight cementing material that could meet the criteria established for geothermal cements. The most important of these criteria are a slurry density of &lt;1.2 g/cc at 25.degree. C., a 300.degree. C.-24 hour compressive strength of &gt;1000 psi (6.9 MPa), and a bulk density of &lt;1.0 g/cc.